Sleep mask



FIG. 1 is a front elevational view of a sleep mask showing my new design;

FIG. 2 is a right side elevational view, thereof;

FIG. 3 is a left side elevational view, thereof;

FIG. 4 is a rear elevational view, thereof;

FIG. 5 is a top plan view, thereof;

FIG. 6 is a bottom plan view, thereof. 

The ornamental design for a sleep mask, as shown and described. 